The apicomplexan protozoans Cryptosporidium parvum and Toxoplasma gondii are ubiquitous parasites that infect humans and domesticated animals. Recently C. hominus was recognized to be distinct from C. parvum, and does not appear to infect domesticated animals, but rather appears limited to human infections. C. parvum and C. hominus are infectious parasites of major health concern in humans as they are a common cause of illness transmitted by water. (See White A C. Chapter 280: Cryptosporidiosis (Cryptosporidium hominis, Cryptosporidium parvum, and Other Species) in Mandell, Bennett, & Dolin: Principles and Practice of Infectious Diseases, 6th ed. Publ: Churchill Livingston (2005)) C. parvum and C. hominus infections result in debilitating diarrhea that can be life-threatening in immunocompromised patients.
Recent studies have implicated Cryptosporidium spp. in around 15-20% of childhood diarrheal disease in the developing world. (See Samie et al., Cryptosporidium species: preliminary descriptions of the prevalence and genotype distribution among school children and hospital patients in the Venda region, Limpopo Province, South Africa. Exp. Parasitol. 114, 314-322 (2006); and Ajjampur et al., Closing the diarrhea diagnostic gap in Indian children by the application of molecular techniques. J. Med. Microbiol. 57, 1364-1368 (2008)) Currently, nitazoxanide is the only approved therapy for cryptosporidiosis but it is expensive and has not been shown to be effective in treating immunocompromised hosts. T. gondii may be the most common infectious eukaryotic parasite in humans, based on serosurveys. (See Montoya et al., Chapter 276: Toxoplasma gondii in Mandell, Bennett, & Dolin: Principles and Practice of Infectious Diseases, 6th ed. Publ: Churchill Livingston (2005)) Transmitted primarily through undercooked meat or accidental ingestion of cat feces, T. gondii infection presents major health concerns in immunocompromised hosts, where it causes toxoplasmic encephalitis, and in pregnancy, where it can result in severe birth defects or miscarriage. Sulfadiazine and pyrimethamine are the current therapies for toxoplasmosis, but they can cause nephrotoxicity, rash, and additional complications in pregnancy. Thus, new therapies for treating infections caused by both parasites are greatly needed.
In T. gondii, calcium-regulated signaling is associated with a number of cellular functions such as secretion, gliding motility and host cell invasion. (See Nagamune and Sibley, Comparative genomic and phylogenetic analyses of calcium ATPases and calcium-regulated proteins in the apicomplexa. Mol. Biol. Evol. 23, 1613-1627 (2006); and Billker et al., Cell Host Microbe. 2009 Jun. 18; 5(6):612-22. Calcium-dependent signaling and kinases in apicomplexan parasites) The proper control of intracellular calcium levels is important for host cell invasion and T. gondii use several mechanisms for the uptake and release of calcium. Furthermore, this organism contains specialized calcium-regulated signaling enzymes, including a unique family of calcium-dependent protein kinases (CDPKs) which are present in plants, ciliates and green algae but not in animals. (See Doerig et al., Protein kinases as targets for antimalarial intervention: kinomics, structure-based design, transmission-blockade, and targeting host cell enzymes. Biophysica et Biochimica Acta—Proteins and Proteomics 1754, 132-150 (2005)) These kinases are believed to be mediators of secretion, invasion, and gliding motility. (See Nagamune and Sibley L D, supra; Billker et al., supra; and Kieschnick et al., C. Toxoplasma gondii attachment to host cells is regulated by a calmodulin-like domain protein kinase. J. Biol. Chem. 276, 12369-12377 (2001)) T. gondii, C. parvum, and C. hominus are highly related obligate intracellular parasites. While much less is known about the role of calcium signaling in C. parvum and C. hominus, it appears that many calcium-regulated signaling processes are conserved from T. gondii to C. parvum. (See Chen et al., Apical Organelle discharge by Cryptosporidium parvum is temperature, cytoskeleton, and intracellular calcium dependent and required for host cell invasion. Infect. Immun. 72, 6806-16 (2004)) C. parvum and C. hominus also possess CDPKs that are believed to play important roles in calcium-regulated processes and they are virtually identical in these two spp. Thus inhibitors of C. parvum CDPKs would be expected to inhibit C. hominus CDPK.
The roles that CDPKs play in calcium signaling in T. gondii, C. parvum and C. hominus make this family of kinases intriguing targets for the development of anti-parasitic agents.